US3975588A - Acoustic feedback control - Google Patents

Acoustic feedback control Download PDF

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Publication number
US3975588A
US3975588A US05/532,548 US53254874A US3975588A US 3975588 A US3975588 A US 3975588A US 53254874 A US53254874 A US 53254874A US 3975588 A US3975588 A US 3975588A
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Prior art keywords
decoder
coder
signal
pulse
quantizing
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Expired - Lifetime
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US05/532,548
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English (en)
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Jacques A. Besseyre
Christian A. Jacquart
Jean-Louis H. Marijon
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International Business Machines Corp
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International Business Machines Corp
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04M—TELEPHONIC COMMUNICATION
    • H04M9/00—Arrangements for interconnection not involving centralised switching
    • H04M9/08—Two-way loud-speaking telephone systems with means for conditioning the signal, e.g. for suppressing echoes for one or both directions of traffic
    • H04M9/085—Two-way loud-speaking telephone systems with means for conditioning the signal, e.g. for suppressing echoes for one or both directions of traffic using digital techniques

Definitions

  • This invention relates to acoustic feedback control, sometimes called an anti-Larsen device, provided in the telephone extensions of a digital telephone system.
  • the device of this invention is more particularly designed to be installed in the extensions provided with a loud speaker and a microphone, in which the coders used for the analog/digital conversion of the voice signals are of the syllabic compression type.
  • each extension is provided with an analog/digital coder at the microphone output and with a digital/analog decoder at the loud speaker input.
  • the signal issued from the coder output represents the voice signal transmitted on a transmission channel and the signal applied to the decoder input represents the voice signal received from the second transmission channel and thus represents the voice signal applied to the input of this channel.
  • Two measuring circuits are provided at each end, the first one being connected to the coder output and supplying, according to the digital information it receives, an output voltage representing the level of the voice signal on the corresponding channel, the second one being connected to the decoder input and supplying, according to the digital information it receives, an output voltage representing the level of the voice signal on the corresponding channel.
  • a comparator receives these two output voltages and the comparator output signal indicates which channel input is provided with the voice signal with the highest level.
  • a switch is installed at each end on the channel issued from the microphone. This switch, in rest position, operates to disconnect the microphone/coder assembly from the channel and a rest signal is provided on this channel.
  • the comparator output signal indicates that the signal applied to the input of a channel shows a level exceeding the level of the signal applied to the input of the second channel, the corresponding channel switch is closed and the connection is made through this channel. Since, on this time, the second channel is "open", the Larsen effect does not appear.
  • the position of the switches will change as the call proceeds according to the output signal continuously generated by the comparator.
  • the major object of this invention is to provide an anti-Larsen device usable in the same environment as before but not having the above indicated disadvantages. More particularly, the device of this invention includes only a minimum number of components and operates entirely in digital mode.
  • a primary object of this invention is fulfilled by taking into account the inherent characteristics of the syllabic compression coders and decoders used, in general, to code a voice signal.
  • the device of this invention is especially well adapted to use in a telephone system in which the voice signals are transformed into digital information in syllabic compression coders.
  • This coding mode is the mode generally used for voice coding due to the nature of the voice signal which can show important slopes for which the quantizing noise would be important, should a simple coding without compression be used.
  • the compression operating principle is well known in the art and applies to the coding known as the "delta modulation" or to the coding known as the "pulse coded modulation” (PCM). According to this principle, the amplitude of the analog signal is considered. For example, for delta coding, the height of the quantizing step varies in accordance with the signal slope and, for PCM coding, the compression can be carried out after linear coding, the amplitude information being taken from the linear coding.
  • the anti-Larsen device of this invention can be used in a telephone system, in the connections between extensions provided with a loud speaker and a microphone.
  • Each extension of this type includes a microphone connected to a coder as defined above and a loud speaker connected to a decoder.
  • the telephone connection between two extensions will be established through a first channel connecting the microphone of the first extension to the loud speaker of the second extension and a second channel connecting the loud speaker of the first extension to the microphone of the second extension.
  • the amplitude information in the coder of the first channel indicates the signal level at the input of the first channel and the amplitude information in the decoder of the first channel indicates the signal level at the input of this channel.
  • the amplitude information in the coder of the second channel indicates the signal level at the input of the second channel and the amplitude information in the decoder of the second channel indicates the signal level at the input of this channel.
  • a comparator is connected, on the one hand, to the coder, and on the other hand, to the decoder, thus receiving an indication about the level of the signals at the input of the two channels involved in the connection.
  • the comparator When, at one end of the connection, the comparator indicates that the level of the signal emitted on a channel through the coder exceeds the level of the signal received on the other channel through the decoder, one deduces that the subscriber is talking at this end of the connection.
  • the output signal of the comparator indicating this condition acts on an inhibit circuit connected to the decoder placed at this end to prevent the decoder step from increasing.
  • the signal received by the extension at this end will be limited in amplitude, and only the channel through which the voice signals are emitted operates normally.
  • the anti-Larsen circuit will be inhibited when this condition is detected and both channels operate normally.
  • both channels will operate normally
  • the anti-Larsen device at each end of the connection acts on the decoders and the signals on both channels are limited in amplitude at the receiving end;
  • one channel having the higher level will operate normally and the signal received on the other channel will be limited in amplitude.
  • FIGS. 1a and 1b are schematic views of a compression coder and of an expansion decoder of the syllabic type, respectively.
  • FIG. 2 ia a general view of a circuit according to the principle of this invention.
  • FIG. 3 shows a circuit employed in computing the step in the coders and decoders of FIG. 2.
  • FIG. 4 shows the anti-Larsen device according to a preferred embodiment of this invention.
  • a delta coder includes a comparator COMP. receiving the analog signal to be coded on a first input EA and a locally reconstructed analog signal on its second input ER.
  • Signal S ⁇ at the output of the comparator is applied to an analysis circuit P which, according to the digital information, computes the compression coder step so that the reconstructed signal is as close as possible to the real signal to reduce the quantizing noise to a minimum.
  • This step magnitude information is applied to an integrator INT which will integrate, according to the bit at the comparator output, a positive (bit I) or negative (bit 0) constant current for a time determined by the step magnitude. For instance, if the sampling period is divided into sixteen parts, to vary the step, an integration will be carried out for a time determined by the analysis circuit, corresponding to one of the sixteen parts or more. Therefore, it is seen that the step magnitude information is, in this case, represented by a pulse of variable length.
  • the decoder shown in FIG. 1b includes an integrator INT' receiving on the one hand, the signal coded in S ⁇ and the step magnitude information computed from this signal as previously carried out by circuit P' and supplies reconstructed analog signal SA as an output.
  • a telephone extension A able to operate in two different ways, is used. In the first way, it is associated to a conventional telephone set and, in the second way, it is associated to a loud speaker and to a microphone. These two operating modes are selected through a switch CT placed either in position T (telephone) or in position MH (microphone, loud speaker).
  • microphone M is connected through amplifier A1 to a coder C and loud speaker HP is connected through an amplifier A2 to a decoder D.
  • these coders and decoders are of the syllabic compression type and, therefore, are provided with a variable quantizing step. Therefore, they include means to compute the step to be used.
  • An example of a circuit able to be used to compute the step in a delta coder will be described later with reference to FIG. 3.
  • a comparison circuit 3 compares the step lengths used in the coder and in the decoder, and supplies the result of the comparison on a line 4 which is connected to the decoder.
  • extension B is shown in FIG. 2; the elements of this extension bear the same references as the ones of extension A, but prime symbols (') are added.
  • microphone M of extension A is connected through a switching network not shown on the figure, to loud speaker HP' of extension B through channel 1 and microphone M' of extension B is connected to loud speaker HP of extension A through channel 2.
  • decoder D decodes the signal issued from coder C' and consequently it has the same step as coder C'.
  • the step determining circuit as shown in FIG. 3 includes a n-stage register 5 receiving signals S ⁇ issued from the comparator shown in FIG. 1.
  • n 4.
  • the four outputs of register 5 are applied to a binary comparator 6 which supplies an output if the four bits contained in the register are identical and equal either to 1 or to 0.
  • the signal on output line 7 of comparator 6 is applied on the one hand, to reset input R of a counter 8 and to incrementing input I of a reversible binary counter 9.
  • Both counters 8 and 9 are controlled by a signal of frequency F equal, for example, to 64 KHz which is the delta coding sampling frequency supplied by clock H.
  • F the signal of frequency
  • the contents of counter 9 is decreased when it receives a signal on input DI, which occurs when counter 8 has counted N sampling periods during which there was no signal applied to input R.
  • N 16.
  • the outputs of binary counter 9 are applied to a decimal binary decoder 10 supplying the number of the step to be used as an output.
  • An integration control device 11 which can be a counter/comparator assembly controlled by a signal Hk having a frequency which is a multiple of the sampling frequency, supplies, according to the step number information, a pulse of an appropriate duration which is a fraction of the sampling period as will be defined later.
  • This pulse is applied to an integrator which integrates a positive or negative constant current in accordance with the value of the bit of the delta coding, during the up level of the pulse and will maintain the obtained value during the whole sampling period.
  • the pulse duration can be computed for each step in the following way:
  • step 16 will have a value of 32 decibels, which corresponds to a decimal value Y 16 which can be computed from the following equation:
  • control circuit will supply for each step number a pulse the up level of which has the appropriate duration obtained by the previous calculations.
  • the step has a minimum value and during the first four sampling periods this step is used. If the four bits obtained during these four periods are equal, a signal appears at the output of binary comparator 6, said signal on the one hand, incrementing the contents of counter 9 so that the value of the step increases and, on the other hand, resetting counter 8.
  • step will increase on each sampling period.
  • counter 8 will start to count and the step will not vary any more. If the counter counts 16 clock periods without it being reset, a signal will appear at input DI of counter 9 and the contents of counter 9 will begin to decrease, which causes the step to decrease.
  • channels 1 and 2 through which the connection between two extensions A and B is made include the same elements as in FIG. 2. Only the comparing circuit of extension A is shown in more detail.
  • the coders and decoders include a step determining circuit as shown in FIG. 3 and only the elements of this circuit required to understand the invention are shown in the coders and decoders.
  • the anti-Larsen device includes at each extension a latch FF of type D used to compare the pulse lengths representing the step values and an AND logic circuit 12 receiving on one of its inputs, the signal on line 7 and on its second input, the result of the comparison.
  • the type D latches are well known in the art and they include two inputs currently referenced C and D, a reset input R and two outputs Q and Q. When two signals are applied to both inputs C and D, the signal from output Q has the level of the signal applied to input D when the level of the signal in C moves from 0 to 1.
  • the latch receives onto its input C, the reversed decoder step length pulse and onto its input D, the coder step length pulse. Since the coder and decoder in an extension are synchronized, the forward edges of this pulses coincide.
  • Another logic circuit including an AND circuit 13 and OR circuits 14 and 15 allow the carrying out of the comparison only when the coder and decoder steps exceeds a threshold value chosen, in a preferred embodiment, as equal to step No. 8 to avoid any ambiguity when the levels of the signals on both channels are low, in which case both channels operate normally.
  • a third logic circuit including an AND circuit 16 and an OR circuit 17 is provided at the output of latch FF. The operation of this circuit will be described later. Its function is to supply as an output the step duration pulse applied to the coder integrator to prevent the coder step duration from becoming lower than a limit value chosen, in a preferred embodiment, as equal to 4, value which corresponds to the voice and noise threshold to limit the channel gain in certain cases to be specified later.
  • binary counter 9 decimal binary decoder 10
  • integration command circuit 11 are the elements of the step determining circuit shown in FIG. 3. There has added to these elements, suffix 1 in the coder to indicate that said coder is on channel 1 and suffix 2 in the decoder to indicate that said decoder is on channel 2 for extension A shown in the figure.
  • extension A coder step represents the level of the signal at the microphone output, therefore the level of the signal on channel 1
  • extension A decoder step is equal to the one of the coder of extension B connected to A, and therefore represents the level of the signal at the input of channel 2.
  • AND circuit 16 receiving on one of its inputs, signal 1 issued from output Q and the pulse of a duration corresponding to step 4 memorized in circuit 11-1, will deliver this pulse as an output. Therefore, OR circuit 17 will supply, as an output, either this pulse if the pulse on S-1 has a lower duration, or the pulse on S-1 if the step exceeds 4. In this way, the pulse at the output of OR circuit 17 will have always a value exceeding or equal to the one corresponding to step 4. The step used for coding will exceed or be equal to step 4, which corresponds to the voice and noise threshold.
  • the latch will be also reset and the decoder of extension B will operate normally. Since the decoders at each end of the connection operate normally, cut-offs at low levels are avoided.
  • the coder step exceeds or is equal to 8 and that it exceeds the decoder step. For example, consider the case in which the coder step is 10 and the decoder step is 8.
  • AND gate 16 is no longer conditioned and the signal at the output of OR circuit 17 is the same as the one on line S-1.
  • the coder step is lower than the decoder step.
  • the step is 8 for the coder and 10 for the decoder.
  • the latch will be set to 0 (the signal on input D being low when the signal on input C moves from 0 to 1).
  • the signal at output Q will be 1 and, consequently, the clock pulses will be applied through AND gate 12 to the counter of the decoder which operates normally.
  • AND circuit 16 is conditioned and the pulse at the output of OR circuit 17 will be the pulse on line S-1 since, in the case taken as an example, the coder step is equal to 8, i.e. above 4.
  • the pulse at the OR circuit 17 output will be the pulse of step 4 duration transmitted to OR circuit 17 by AND circuit 16.
  • the step used in the coder cannot decrease below 4.
  • the accidental noises able to occur at end A are prevented from causing an inversion of the operating mode.
  • extension B the coder step exceeds the decoder step and the anti-Larsen device of this extension will prevent the decoder step from increasing. Then, the signal on channel 1 will be limited in amplitude.
  • the anti-Larsen devices provided at each end of the connection act on the decoders and the signals on both channels are limited in amplitude

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Analogue/Digital Conversion (AREA)
  • Interconnected Communication Systems, Intercoms, And Interphones (AREA)
  • Use Of Switch Circuits For Exchanges And Methods Of Control Of Multiplex Exchanges (AREA)
US05/532,548 1973-12-21 1974-12-13 Acoustic feedback control Expired - Lifetime US3975588A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR73.47136 1973-12-21
FR7347136A FR2255762B1 (cs) 1973-12-21 1973-12-21

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US3975588A true US3975588A (en) 1976-08-17

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US05/532,548 Expired - Lifetime US3975588A (en) 1973-12-21 1974-12-13 Acoustic feedback control

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US (1) US3975588A (cs)
JP (1) JPS5520423B2 (cs)
DE (1) DE2455584C3 (cs)
FR (1) FR2255762B1 (cs)
GB (1) GB1439107A (cs)
IT (1) IT1026644B (cs)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2375786A1 (fr) * 1976-12-27 1978-07-21 Dasy Int Sa Disposition de circuit pour eviter les accrochages
EP0120325A1 (de) * 1983-03-01 1984-10-03 Alcatel N.V. Schaltungsanordnung für einen Freisprechapparat
US4491694A (en) * 1981-10-23 1985-01-01 Michael Harmeyer Telephone to stereo amplifier interface coupling
US4583237A (en) * 1984-05-07 1986-04-15 At&T Bell Laboratories Technique for synchronous near-instantaneous coding
US4696032A (en) * 1985-02-26 1987-09-22 Siemens Corporate Research & Support, Inc. Voice switched gain system
US4726062A (en) * 1985-12-11 1988-02-16 Harris Corporation Duplex hands-free communications circuit
EP0288078A3 (en) * 1987-04-24 1989-05-31 Motorola Inc. Speakerphone using digitally compressed audio to control voice path gain
US5010570A (en) * 1990-02-20 1991-04-23 Motorola, Inc. Method for active feedback suppression
US5228076A (en) * 1989-06-12 1993-07-13 Emil Hopner High fidelity speech encoding for telecommunications systems

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2122851B (en) * 1982-06-10 1986-03-19 Standard Telephones Cables Ltd Loudspeaking telephones

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3562448A (en) * 1968-06-21 1971-02-09 Bell Telephone Labor Inc Common control digital echo suppression
US3673355A (en) * 1970-09-02 1972-06-27 Bell Telephone Labor Inc Common control digital echo suppression
US3735055A (en) * 1971-11-05 1973-05-22 Bell Telephone Labor Inc Method for improving the settling time of a transversal filter adaptive echo canceller
US3821494A (en) * 1972-07-14 1974-06-28 Ibm Digital echo suppressor with direct table look up control by delta coded signals
US3832491A (en) * 1973-02-13 1974-08-27 Communications Satellite Corp Digital voice switch with an adaptive digitally-controlled threshold

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3562448A (en) * 1968-06-21 1971-02-09 Bell Telephone Labor Inc Common control digital echo suppression
US3673355A (en) * 1970-09-02 1972-06-27 Bell Telephone Labor Inc Common control digital echo suppression
US3735055A (en) * 1971-11-05 1973-05-22 Bell Telephone Labor Inc Method for improving the settling time of a transversal filter adaptive echo canceller
US3821494A (en) * 1972-07-14 1974-06-28 Ibm Digital echo suppressor with direct table look up control by delta coded signals
US3832491A (en) * 1973-02-13 1974-08-27 Communications Satellite Corp Digital voice switch with an adaptive digitally-controlled threshold

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
IEEE Transactions On Communication, Dec. 1972, A Digital Echo Suppressor For Satellite Circuit, pp. 1176-1181 *

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2375786A1 (fr) * 1976-12-27 1978-07-21 Dasy Int Sa Disposition de circuit pour eviter les accrochages
US4165445A (en) * 1976-12-27 1979-08-21 Dasy Inter S.A. Circuit for preventing acoustic feedback
US4491694A (en) * 1981-10-23 1985-01-01 Michael Harmeyer Telephone to stereo amplifier interface coupling
EP0120325A1 (de) * 1983-03-01 1984-10-03 Alcatel N.V. Schaltungsanordnung für einen Freisprechapparat
US4583237A (en) * 1984-05-07 1986-04-15 At&T Bell Laboratories Technique for synchronous near-instantaneous coding
US4696032A (en) * 1985-02-26 1987-09-22 Siemens Corporate Research & Support, Inc. Voice switched gain system
US4726062A (en) * 1985-12-11 1988-02-16 Harris Corporation Duplex hands-free communications circuit
EP0288078A3 (en) * 1987-04-24 1989-05-31 Motorola Inc. Speakerphone using digitally compressed audio to control voice path gain
US5228076A (en) * 1989-06-12 1993-07-13 Emil Hopner High fidelity speech encoding for telecommunications systems
US5010570A (en) * 1990-02-20 1991-04-23 Motorola, Inc. Method for active feedback suppression
WO1991013508A1 (en) * 1990-02-20 1991-09-05 Motorola, Inc. A method for active feedback suppression

Also Published As

Publication number Publication date
JPS5520423B2 (cs) 1980-06-02
JPS5094802A (cs) 1975-07-28
FR2255762A1 (cs) 1975-07-18
DE2455584A1 (de) 1975-07-03
DE2455584C3 (de) 1978-08-10
DE2455584B2 (de) 1977-12-15
IT1026644B (it) 1978-10-20
GB1439107A (en) 1976-06-09
FR2255762B1 (cs) 1976-07-23

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